Currently, various I-type level circuits have been widely applied into multiple fields due to an advantage of high efficiency. FIG. 1 shows a common I-type three-level circuit, including: switching tubes S1, S2, S3 and S4; clamp diodes Dc1 and Dc2; an inductor L; bus capacitors Co+ and Co−; diodes D1, D2, D3 and D4 are external anti-paralleled diodes or body diodes of the switching tubes S1, S2, S3 and S4, respectively; capacitors Coss1, Coss2, Coss3 and Coss4 are equivalent junction capacitors of the switching tubes S1, S2, S3 and S4, respectively; and capacitors Cdc1 and Cdc2 are equivalent junction capacitors of the clamp diodes Dc1 and Dc2, respectively, wherein:
the switching tubes S1, S2, S3 and S4 are connected in sequence to construct a high-frequency bridge arm, the bus capacitors Co+ and Co− are connected in series, and then connected in parallel between two terminals of the high-frequency bridge arm, the two terminals of the high-frequency bridge arm are direct-current (DC) connection terminals of the I-type three-level circuit. A middle point of the high-frequency arm bridge is a terminal, connected to the inductor L, of a connection node of the switching tubes S2 and S3, the other terminal of the inductor L is an alternating-current (AC) connection terminal of the I-type three-level circuit. A connection node of the switching tubes S1 and S2 is connected to a connection node of the switching tubes S3 and S4 via the clamp diodes Dc1 and Dc2 connected in series. A connection node of the clamp diodes Dc1 and Dc2 and a connection node of the bus capacitors Co+ and Co− are connected together, and then are earthed.
Specifically, the switching tubes S1, S2, S3 and S4 may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), as shown in FIG. 2. The switching tubes S1, S2, S3 and S4 may be a controllable switching tube such as an Insulated Gate Bipolar Transistor (IGBT), which is not illustrated here.
FIG. 1 shows an I-type three-level circuit. In a positive half cycle of an AC connection terminal voltage UN, the switching tube S1 is a freewheeling switching tube, the switching tube S2 is in ON state, the switching tube S3 is a main switching tube, the switching tube S4 is in OFF state. In a negative half cycle of the AC connection terminal voltage UN, the switching tube S1 is in OFF state, the switching tube S2 is a main switching tube, the switching tube S3 is in ON state, and the switching tube S4 is a freewheeling switching tube.
In conventional technology, a common control method for the I-type three-level circuit is a control of inductor current continuous conduction mode such as Critical Conduction Mode (CRM) control. However, such control method can not realize zero-voltage turn on of the switching tube of the high-frequency arm bridge, which results in great circuit loss.